1. Field of the Invention
The present invention pertains to an OFDM receiving apparatus for receiving transmission signals using an Orthogonal Frequency Division Multiplex (OFDM) modulation system and a method of demodulation in the OFDM receiving apparatus. In particular, the present invention pertains to a technique for increasing the receiving performance of an OFDM demodulation.
2. Description of the Related Art
For transmission of audio signals and video signals, the development of digital-modulation based transmission systems has recently become very active. In digital terrestrial broadcasting, in particular, Orthogonal Frequency Division Multiplex (OFDM) has been receiving great attention because of features such as its robustness against multipath interference and a higher efficiency in frequency usage.
As several thousands of sub-carriers are multiplexed into transmitted signals in the OFDM modulation system, the transmitted signals are susceptible to carrier phase noise. In the consumer-oriented OFDM receiving apparatus, in particular, the phase noise characteristics of a tuner are an important issue because of limited costs. The effect of the phase noise becomes evident as Common Phase Error (CPE), which shifts the phases of all of the sub-carriers at the same angle.
Japanese Patent Laid Open Publication (Kokai) No. 2000-286819 discloses an OFDM receiving apparatus compensating for the CPE using a known pilot signal assigned to the predetermined sub-carriers so that the receiving performance will not be deteriorated by the phase noise of a tuner.
In the above-described OFDM receiving apparatus, the known pilot signals are inserted into the specific sub-carriers at a transmission side, the difference between the known pilot signal and the received signals are determined at a reception side, and in an internal-symbol average circuit, the average is obtained for the phase error of each sub-carrier for each OFDM symbol. At this time, setting a threshold value according to a receiving level, and obtaining the average in the internal-symbol of the phase error only regarding sub-carriers with a receiving level higher than the threshold value, when multipass interference exists, the accuracy of the phase error information can be insured. And then, phase rotation of the received signals can be performed using the phase error information so that the phase noise of the tuner may be compensated for.
The sub-carriers used for the phase error detection are judged only on the receiving level in the above OFDM receiving apparatus, although it is effective in deleting the influence of the sub-carriers in which the receiving level fell due to multipass interference. However, in the above OFDM receiving apparatus, since the receiving level of the sub-carriers receiving interference reaches a high level when the same channel interference or Continuous Wave (CW) interference of an analog television signal exists, the sub-carriers of the pilot signal receiving such interference will not be able to be eliminated from the sub-carriers used for the phase error detection, and the accuracy of phase error information will deteriorate.
Moreover, although there is a method of performing timing synchronous reproduction and Automatic Frequency Control (AFC) of a carrier required for the OFDM demodulation by detecting the correlation of the guard period inserted in the OFDM signals, when the same channel interference of the analog television signals or the CW interference exist, the accuracy of the correlation detection value deteriorates, and neither the timing synchronization nor the synchronization of the AFC of carrier is obtained, and receiving becomes impossible.